1. Field of the Invention
The present invention relates to an image forming material using a novel color-change material, a lithographic printing plate precursor having an image forming layer using the color-change material, and a lithographic printing method using the lithographic printing plate precursor. More specifically, the present invention relates to an image forming material capable of forming an image by scanning an infrared laser based on digital signals of a computer or the like and ensuring a printout image with good visibility, a lithographic printing plate precursor having a layer using the color-change material, and a lithographic printing method using the printing plate precursor.
2. Background Art
The lithographic printing plate in general consists of a lipophilic image area of accepting ink in the printing process and a hydrophilic non-image area of accepting a fountain solution. In conventional plate-making, a lithographic printing plate is usually produced by subjecting a PS plate comprising a hydrophilic support having provided thereon a lipophilic photosensitive resin layer (image forming layer) to mask exposure through a lith film, and dissolving and thereby removing the unnecessary portion of the image forming layer with a developer.
In recent years, an image is electronically processed as digital information, stored and output by a computer. Accordingly, in the image formation processing according to the digital image information, an image is preferably formed directly on a lithographic printing plate precursor by scan exposure using high-directivity actinic radiation without intervention of a lith film. Such a technique of plate-making a printing plate based on distal image information without intervention of a lith film is called “computer-to-plate (CTP)”.
When the conventional plate-making method using a PS plate for producing a printing plate is intended to implement by the CTP technique, a problem arises that the wavelength region of laser light does not agree with the photosensitive wavelength region of photosensitive resin.
Also, conventional PS plates indispensably requires a step of dissolving and removing the unnecessary portion of the image forming layer (development processing) after exposure. Moreover, an after-processing step of washing the developed printing plate with water or treating it with a rinsing solution containing a surfactant or with a desensitizing solution containing gum arabic or a starch derivative is also necessary. A great problem to be solved of conventional PS plates is that such additive wet processing is indispensable. Even when the first half (image formation processing) of the plate-making process is streamlined by the above-described digital processing, if the latter half (development processing) is a cumbersome wet processing, the effect by the streamlining is not satisfactorily exerted.
In particular, consideration for global environment recently becomes a great concern in the entire industry. The wet after-processing is preferably made simple or changed to a dry processing also in view of environmental consideration.
As one of the methods for dispensing with the processing step, a method called on-press development is known, where an exposed printing plate precursor is loaded on a cylinder of a printing press and a fountain solution and an ink are supplied while rotating the cylinder, thereby removing the unnecessary portion of the image forming. Namely, this is a system of loading a printing plate precursor as it is on a printing press after exposure and completing the processing during the normal printing process.
The lithographic printing plate precursor suited to such on-press development is required to have an image forming layer soluble in a fountain solution or an ink solvent and moreover, has an appropriate bright room handleability for enabling development on a printing press placed in a bright room.
These requirements cannot be substantially satisfied by conventional PS plates.
In order to satisfy these requirements, a lithographic printing plate precursor where an image forming layer comprising a hydrophilic binder polymer having dispersed therein hydrophobic thermoplastic polymer particles is provided on a hydrophilic support has been proposed (see, for example, Patent Document 1: Japanese Patent No. 2,938,397). In the plate-making, this lithographic printing plate precursor is exposed by an infrared laser to cause coalescence (fusion) of hydrophobic thermoplastic polymer particles by the effect of heat generated resulting from light-to-heat conversion and thereby form an image, and then loaded on a cylinder of a printing press, and at least either a fountain solution or an ink is supplied, whereby on-press development can be performed. This lithographic printing plate precursor has its photosensitive region in the infrared region and therefore, ensures handleability in a bright room.
However, the image formed by the coalescence (fusion) of hydrophobic thermoplastic polymer fine particles is insufficient in the strength and the press life as a printing plate has a problem.
Also, a lithographic printing plate precursor comprising, in place of the thermoplastic fine particle, a microcapsule enclosing a polymerizable compound has been proposed (see, for example, Patent Documents 2 to 7: JP-A-2000-211262 (the tem “JP-A” as used herein means an “unexamined published Japanese patent application”), JP-A-2001-277740, JP-A-2002-29162, JP-A-2002-46361, JP-A-2002-137562, JP-A-2002-326470; respectively). The printing plate precursor proposed here is advantageous in that the polymer image formed by a reaction of the polymerizable compound has higher strength than that of the image formed by the fusion of fine particles.
Furthermore, the polymerizable compound has high reactivity and therefore, many methods have been proposed for isolating the polymerizable compound by using a microcapsule (see, for example, Patent Documents 2 to 7). For the shell of the microcapsule, use of a thermally decomposable polymer has been proposed.
However, in the lithographic printing plate precursors not involving a development processing step before printing described in Patent Documents 2 to 7, the image formed by laser exposure can be hardly confirmed on the printing plate. Therefore, loading on a printing press encounters a problem that whether an objective image is recorded on the printing plate or what color ink is assigned to the plate cannot be confirmed or the top and bottom of the printing plate is wrongly disposed and the miss-operation is not found until the printing is performed. Accordingly, it is demanded to more enhance the visibility of the image after exposure.